I. Field of the Invention
The invention is concerned with an absorption filter for the purification of gas or air streams, especially such gas or air streams containing toxic or radioactive contaminants.
II. Description of the Prior Art
Absorption filters of this type are familiar in many different design versions. They consist of at least one filter chamber, which is characterized by featuring gas-permeable walls, arranged traverse to the direction of flow. The filter chambers conventionally consist of a prismatic housing, capable of being charged from the top with a granular absorption medium. The absorption medium, in turn, can be removed from the bottom of the housing after having reached a certain degree of saturation, while fresh substrate is being added from the top. Thus, the filter chamber can remain in virtually uninterrupted service. The filter chamber housing is equipped on one side with an adapter for the contaminated air and on the other side with an exit pipe for the purified air. The feed pipe and exit pipe are joined to each other by a bypass line which is parallel to the absorption medium of the filter chamber and is also filled with the absorption substrate. This bypass filter section allows for monitoring the degree of saturation of the absorption substrate contained in the filter chamber by removing the filter medium from the bypass filter section.
The contaminants carried by the gas or air stream, passing through the absorption filter, are progressively removed from the supply side of the chamber to the exit side. Since the contaminated gas or air streams always contain several different types of gaseous contaminants (which are absorbed by the filter medium with markedly different degrees of effectiveness), it is of extreme importance, especially in the presence of toxic or radioactive contaminants in gaseous form, to absorb these materials first in the filter medium, both with respect to time and location. It is also important to remove the more difficultly absorbed gaseous contaminants at regions which are closer to the exit side of the purified gas stream. It is especially important that the effectiveness of removal of these contaminants is not impaired in this region by the presence of the more easily absorbed contaminants. In other words, care must be taken to preserve a region of sufficient capacity near the pure air exit side for the absorption of gaseous contaminants in which none of the more easily absorbed contaminants are present.
In order to provide for a solution to this problem, an absorption filter has been designed (West German Patent No. DT-OS No. 2,239,827), which features an additional gas-permeable divider in the approximate center of the filter chamber, thus providing for two sequential chambers in series within one housing. The filter medium in both chambers is in direct contact with the central gas-permeable divider section. Both chamber sections can be individually and separately charged with the filter medium. The advantage of this design feature is to be found in the fact that the filter medium from the first section, initially exposed to the contaminated gas stream, can be seperately removed and replaced by fresh substrate without also having to empty the second chamber where the filter medium has not yet reached a degree of saturation requiring recharge. The exact determination of the time when replacement of the filter medium in the first chamber section is required can, however, not be made in the case of this state of the art design version, thus allowing for the danger that the chamber section intended for the absorption of the more difficultly removed contaminants will be contaninated from the direction of the saturated previous chamber section with the more easily absorbed contaminants and, thus, no longer allowing for an unambiguous guarantee that the more difficultly absorbed gaseous contaminants can be absorbed in the chamber section situated close to the pure air exit line. For the determination of the degree of saturation for this design version, only the familiar bypass filter section is available, which runs parallel to the adjoining chamber sections and bridges them and, therefore, is unable to provide any information concerning the degree of saturation of the chamber section close to the feed air side of the total assembly. The bypass filters serve as control filters and are arranged parallel to the filter housing and are also filled with filter medium. The bypass filter is exposed to the same gas stream; their filtration thickness is identical to the filter medium contained in the main chamber. By removing the bypass filter section, followed by analysis of the absorption medium, conclusions can be drawn with respect to the saturation state of the filter medium in the main chamber. This method is awkward, and information concerning the saturation of the filter medium with the more easily absorbed contaminants can only be obtained, if at all, with the time delay inherent in dismantling the bypass section and carrying out the analysis required. During this period the more easily absorbed contaminants could saturate the chamber section close to the contaminated air entry side, break through into the adjoining chamber section, and contaminate this half of the total assembly by enrichment with the easily absorbed components and, thus, impairing the trouble-free absorption of the more difficultly absorbed, toxic, gaseous contaminants.